Classification of RNase enzymes:

Ribonucleases can be divided into Endoribonucleases and Exoribonucleases, and comprise several sub-classes within the EC 2.7 (for the phosphorolytic enzymes) and 3.1 (for the hydrolytic enzymes) classes of enzymes.

Major types of Endoribonucleases:

RNase A is an RNase that is commonly used in research. RNase A (e.g., bovine pancreatic ribonuclease A) is one of the hardiest enzymes in common laboratory usage; one method of isolating it is to boil a crude cellular extract until all enzymes other than RNase A are denatured. It is specific for single-stranded RNAs. It cleaves the 3′-end of unpaired C and U residues, ultimately forming a 3′-phosphorylated product via a 2′,3′-cyclic monophosphate intermediate.

RNase H is a ribonuclease that cleaves the RNA in a DNA/RNA duplex to produce ssDNA. RNase H is a non-specific endonuclease and catalyzes the cleavage of RNA via a hydrolytic mechanism, aided by an enzyme-bound divalent metal ion. RNase H leaves a 5′-phosphorylated product.

RNase I cleaves 3′-end of ssRNA at all dinucleotide bonds leaving a 5′-hydroxyl, and 3′-phosphate, via a 2′,3′-cyclic monophosphate intermediate.

RNase III is a type of ribonuclease that cleaves rRNA (16s rRNA and 23s rRNA) from transcribed polycistronic RNA operon in prokaryotes. It also digests double strands RNA (dsRNS)-Dicer family of RNAse, cutting pre-miRNA (60–70bp long) at a specific site and transforming it in miRNA (22–30bp), that is actively involved in the regulation of transcription and mRNA life-time.

RNase L is an interferon-induced nuclease that, upon activation, destroys all RNA within the cell

RNase P is a type of ribonuclease that is unique in that it is a ribozyme – a ribonucleic acid that acts as a catalyst in the same way as an enzyme. Its function is to cleave off an extra, or precursor, sequence on tRNA molecules. RNase P is one of two known multiple turnover ribozymes in nature. A form of RNase P that is a protein and does not contain RNA has recently been discovered.

RNase PhyM is sequence specific for single-stranded RNAs. It cleaves 3′-end of unpaired A and U residues.

Major categories of Exoribonucleases:

Polynucleotide Phosphorylase (PNPase) functions as an exonuclease as well as a nucleotidyltransferase.

RNase PH functions as an exonuclease as well as a nucleotidyltransferase.

RNase II is responsible for the processive 3′-to-5′ degradation of single-stranded RNA.

RNase R is a close homolog of RNase II, but it can, unlike RNase II, degrade RNA with secondary structures without help of accessory factors.

RNase D is involved in the 3′-to-5′ processing of pre-tRNAs.

RNase T is the major contributor for the 3′-to-5′ maturation of many stable RNAs.

Oligoribonuclease degrades short oligonucleotides to mononucleotides.

Exoribonuclease I degrades single-stranded RNA from 5′-to-3′, exists only in eukaryotes.

Exoribonuclease II is a close homolog of Exoribonuclease I.

Function:

All organisms studied contain many RNases of many different classes, showing that RNA degradation is a very ancient and important process.

As well as cleaning of cellular RNA that is no longer required, RNases play key roles in the maturation of all RNA molecules, both messenger RNAs that carry genetic material for making proteins, and non-coding RNAs that function in varied cellular processes.

In addition, active RNA degradation systems are a first defense against RNA viruses, and provide the underlying machinery for more advanced cellular immune strategies such as RNAi.